In certain instances, it is necessary to measure the concentration of particular substances in a person's bloodstream. Commonly used test procedures for measuring such concentrations are invasive, requiring the drawing of blood. This can be particularly unpleasant for individuals who need to obtain concentration measurements at frequent intervals. For example, diabetic patients need to monitor the levels of glucose in their bloodstream and are required to undergo such invasive measurement procedures on a daily basis, often several times a day. Typically, the measuring is done through a finger prick to draw blood, which is placed on a test strip that is then inserted into a glucose monitoring device.
To avoid the problems of invasive testing, non-invasive methods of measuring the concentration of blood stream components of interest, such as glucose, have been developed. Glucose and certain other compounds are known as “optically-active” compounds. As used herein, the term “optically-active” refers to those compounds that rotate polarized light when it is passed through solutions containing the compounds. Optical activity is also associated with isomers that are identical in chemical formula and structure, but whose atoms differ in spatial orientation such that members of the pairs are mirror images of one another. If both members of an optically-active pair of isomers are present in equal proportions in a mixture, the mixture is called “racemic,” and it will not exhibit a net rotation of polarized light, as the rotary effects of each isomer will cancel each other. However, as is known to those skilled in the art, in mammals, glucose and certain other optically active substances are only present in the form of one of their optically-active isomers, thereby avoiding the rotary cancellation caused by racemic mixtures.
It is desirable to exploit the optical activity of optically active substances such as glucose as a means of non-invasively measuring their concentrations in humans. The rotation of plane polarized light is known to be proportional to the concentration of an optically-active substance in a solution through which the light passes, according to the following relationship:α=[α]D[C]lwherein C is the concentration of the optically-active substance, l is the optical path length (i.e., the length of fluid through which the plane polarized light passes) and [α]D is the specific rotation, a parameter specific to the optically-active substance which varies with temperature of the solution and the wavelength of light used.
One location that is suitable for performing non-invasive glucose measurements is the aqueous humor of the eye. The concentration of glucose in the aqueous humor directly relates to the concentration of glucose in the bloodstream. However, the relationship between the concentration of glucose in the aqueous humor and the rotation of polarized light transmitted through the aqueous humor is difficult to use for purposes of determining the concentration of glucose in the bloodstream. In part, this difficulty stems from the fact that accurately measuring the optical path length (l) is difficult in a structure having a geometry as complex as that of an eye. In addition, known techniques for measuring the rotation angle of plane polarized light are difficult to reliably implement outside of a laboratory setting, in particular a setting in which such non-invasive testing will be performed by the patient or by a technician. As a result, a need has developed for a method and system that address the foregoing problems.